MAC seminar

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MAC seminar

  1. 1. Comparison of MAC protocols in wired systems and wireless systems Mridula Sharma 19.07.2010 Comparison of MAC protocols in wired systems and wireless systems
  2. 2. Outline <ul><li>Introduction </li></ul><ul><li>Multiple Access Protocols </li></ul><ul><li>Wired Network </li></ul><ul><li>Wireless Networks </li></ul><ul><li>Conclusion </li></ul><ul><li>References </li></ul>Comparison of MAC protocols in wired systems and wireless systems
  3. 3. Introduction <ul><li>Large class of networks is built on top of broadcast channels </li></ul><ul><li>Problem : if you’re sharing a channel, then two stations may decide to start frame transmission at the same time </li></ul><ul><ul><li>Frame collision , which means rubbish on the wire </li></ul></ul><ul><li>Solution : Allocate the channel to one of the competing stations </li></ul><ul><li>Allocation can be of two types: </li></ul><ul><ul><li>Static: FDM and TDM </li></ul></ul><ul><ul><li>Dynamic </li></ul></ul>Comparison of MAC protocols in wired systems and wireless systems <ul><ul><li>Protocol to determine who goes next on a multi-access channel belongs to the </li></ul></ul><ul><ul><li>Medium Access Control (MAC) sublayer </li></ul></ul><ul><ul><ul><li>Especially important for LANs </li></ul></ul></ul><ul><ul><ul><li>Provides addressing and channel access control mechanisms </li></ul></ul></ul>
  4. 4. Introduction Comparison of MAC protocols in wired systems and wireless systems
  5. 5. Introduction Functions of MAC layer Comparison of MAC protocols in wired systems and wireless systems <ul><li>On transmission, assemble data into a frame with address and error detection fields </li></ul><ul><li>On reception, disassemble frame, and perform address recognition and error detection </li></ul><ul><li>Govern access to a LAN transmission medium </li></ul>
  6. 6. Multiple Access Protocols <ul><li>ALOHA </li></ul><ul><li>Carrier Sense Multiple Access (CSMA) Protocols </li></ul><ul><li>Collision Free Protocols </li></ul><ul><li>Limited Contention Protocols </li></ul><ul><li>Wavelength Division Multiple Access Protocols </li></ul><ul><li>Wireless LAN Protocols </li></ul>Comparison of MAC protocols in wired systems and wireless systems
  7. 7. ALOHA <ul><li>Radio-based communication network </li></ul><ul><ul><li>Developed in 1970s at the University of Hawaii </li></ul></ul><ul><ul><li>Applicable to any system with competing uncoordinated users </li></ul></ul><ul><li>Two versions </li></ul><ul><ul><li>Pure ALOHA </li></ul></ul><ul><ul><li>Slotted ALOHA </li></ul></ul>Comparison of MAC protocols in wired systems and wireless systems
  8. 8. Pure ALOHA <ul><li>Let users transmit whenever they have data to be sent !!! </li></ul><ul><li>If a collision occurs, finish your current transmission and retry later </li></ul><ul><li>Performance is maximized if all frames have the same size </li></ul><ul><li>Random waiting time required else collisions occur </li></ul>Comparison of MAC protocols in wired systems and wireless systems Fig. : Frames transmitted at completely random intervals
  9. 9. Pure ALOHA- Efficiency <ul><li>Probability that k frames are generated during a given frame time with G attempts per packet time: </li></ul><ul><li>Let P0 be the probability that frame does not suffer from collision </li></ul><ul><ul><li>Throughput , S= G.P0 </li></ul></ul><ul><li>V ulnerable period = 2Tframe </li></ul><ul><li>Probability that a frame will not be damaged during two frame times </li></ul><ul><li>long is: </li></ul>Comparison of MAC protocols in wired systems and wireless systems
  10. 10. Slotted ALOHA Comparison of MAC protocols in wired systems and wireless systems <ul><li>Fr ame transmission can only start at fixed times </li></ul><ul><li>Since the vulnerable period is now halved, </li></ul>Fig. : Throughput versus offered traffic for ALOHA systems
  11. 11. Carrier Sense Multiple Access (CSMA) Comparison of MAC protocols in wired systems and wireless systems <ul><li>CSMA protocols do better than ALOHA : you monitor the channel </li></ul><ul><li>before and/or during transmission </li></ul><ul><li>1-persistent </li></ul><ul><li>Nonpersistent </li></ul><ul><li>p-Persistent </li></ul><ul><li>. </li></ul>
  12. 12. Comparison of MAC protocols in wired systems and wireless systems Fig. : Comparison of channel utilization versus load for various random access protocols
  13. 13. CSMA with Collision Detection (CSMA/CD) Comparison of MAC protocols in wired systems and wireless systems <ul><li>Sense the channel, but immediately stop transmission when you detect a collision </li></ul><ul><li>Saves Channel Bandwidth and time </li></ul><ul><li>Ethernet works like this!!! </li></ul>Fig. : CSMA/CD can be in one of the three states: Contention, Transmission or Idle .
  14. 14. Collision Free Protocols Comparison of MAC protocols in wired systems and wireless systems <ul><li>Bit-Map protocol </li></ul><ul><li>Binary Countdown Protocol </li></ul>
  15. 15. Limited Contention Protocols Comparison of MAC protocols in wired systems and wireless systems <ul><li>Contention systems are good when there’s not much going on </li></ul><ul><li>Collision-free systems are good when there’s generally a lot of traffic </li></ul><ul><li>What we really want is the contention strategy during light loads , and </li></ul><ul><li>collision-free strategy during rush hours !!! </li></ul><ul><li>Solution: Limited Contention Protocols </li></ul><ul><li>Dynamically regulate the number of competing stations during a contention period. </li></ul><ul><li>If there’s not much traffic, the first station will be immediately </li></ul><ul><li>allowed to transmit a frame. </li></ul><ul><li>With a lot of traffic, the strategy reduces to the bit-map protocol. </li></ul>
  16. 16. Limited-Contention Protocols The Adaptive Tree Walk Protocol Comparison of MAC protocols in wired systems and wireless systems
  17. 17. Wavelength Division Multiple Access Protocols Comparison of MAC protocols in wired systems and wireless systems <ul><li>If you have a lot of bandwidth, just divide the channel into sub-channels, and dynamically allocate the sub-channels !!! </li></ul><ul><ul><ul><li>used in fiber optics </li></ul></ul></ul>Fig. : Wavelength Division Multiple Access
  18. 18. Ethernet Comparison of MAC protocols in wired systems and wireless systems <ul><li>CSMA/CD based </li></ul><ul><ul><li>sense the channel, wait until idle, and backoff if collision </li></ul></ul><ul><li>Exponential Backoff </li></ul><ul><li>Near implementation of IEEE 802.3 protocol </li></ul>
  19. 19. 802.3 Frame Layout Comparison of MAC protocols in wired systems and wireless systems <ul><li>Preamble : Seven times 10101010 is used to synchronize the receiver’s clock with that of the sender </li></ul><ul><li>Start : Just a delimiter to tell that the real info is now coming </li></ul><ul><li>Address : Generally 48-bit fields. Leftmost bit indicates ordinary or group addresses (multicast / broadcast). Second bit indicates global or local address </li></ul><ul><li>Length : Ranges from 0-1500. Frames should always be at least </li></ul><ul><li>64 bytes </li></ul><ul><li>Pad : used to fill out the frame to the minimum size </li></ul><ul><li>Checksum : Calculated over the data field. CRC-based </li></ul>
  20. 20. Wireless Networks Comparison of MAC protocols in wired systems and wireless systems <ul><li>Often, there are a number of base stations (a.k.a. access points) connected through guided media </li></ul><ul><li>Nodes can also group together to form an ad hoc network </li></ul>
  21. 21. Wireless LAN Requirements Comparison of MAC protocols in wired systems and wireless systems <ul><li>Throughput </li></ul><ul><li>Number of Nodes </li></ul><ul><li>Connection to backbone LAN </li></ul><ul><li>Service Area </li></ul><ul><li>Battery Power Consumption </li></ul><ul><li>Transmission Robustness and Security </li></ul><ul><li>Collocated Network Operation </li></ul><ul><li>License-free Operation </li></ul><ul><li>Handoff/ roaming </li></ul><ul><li>Dynamic Configuration </li></ul>
  22. 22. Problems Comparison of MAC protocols in wired systems and wireless systems <ul><li>There can be subtle interference: </li></ul><ul><li>Issue (a) </li></ul><ul><li>How can C be prevented from trying to transmit something to B ? </li></ul><ul><ul><ul><li>in that case it will ruin any receipt by B </li></ul></ul></ul><ul><ul><ul><li>hidden station problem </li></ul></ul></ul><ul><li>Issue (b) </li></ul><ul><li>How can we tell C that it is allowed to transmit to D, because this will not interfere with the communication from B to A ? </li></ul><ul><ul><ul><li>exposed station problem </li></ul></ul></ul>
  23. 23. Multiple Access with Collision Avoidance (MACA) Comparison of MAC protocols in wired systems and wireless systems <ul><li>A first sends a Request To Send (RTS) </li></ul><ul><li>B answers with a Clear To Send (CTS) </li></ul><ul><li>C hears only RTS and can freely transmit, knowing it will not interfere with A’s transmission- solves exposed station problem </li></ul><ul><li>D hears only the CTS and keeps still for otherwise it would </li></ul><ul><li>interfere with B’s reception- solves hidden station problem </li></ul>
  24. 24. MACA for Wireless (MACAW) Comparison of MAC protocols in wired systems and wireless systems <ul><li>G ets rid of Ethernet like unfairness associated with binary exponential backoff algorithms !!! </li></ul><ul><li>Acknowledges the importance of link layer acknowledgements </li></ul><ul><ul><li>Makes the protocol from RTS- CTS-Data to RTS-CTS-Data-ACK </li></ul></ul><ul><li>Significantly complex protocol </li></ul><ul><ul><li>Performance loss with lightly loaded channel </li></ul></ul><ul><ul><li>Better throughput and fairer allocation in presence of high loads </li></ul></ul>
  25. 25. Wireless LANs MAC Sublayer Comparison of MAC protocols in wired systems and wireless systems <ul><li>Problem : How do we solve the hidden / exposed station problem ? </li></ul><ul><li>One way or the other, stations should not be allowed to continuously interfere with each other’s transmissions </li></ul>
  26. 26. Wireless LANs MAC Sublayer Comparison of MAC protocols in wired systems and wireless systems <ul><li>IEEE 802.11 provides two methods to deal with this problem: </li></ul><ul><ul><ul><li>Distributed coordination : let the stations figure it out by using a </li></ul></ul></ul><ul><li>collision avoidance protocol (CSMA/CA) </li></ul><ul><ul><ul><li>Point coordination : there’s a central base station that controls who </li></ul></ul></ul><ul><li>goes first- mostly used </li></ul><ul><li>Collision detection is hard to implement on the wireless medium </li></ul><ul><ul><ul><li>radios cannot transmit and receive on the same frequency (half-duplex channels) </li></ul></ul></ul><ul><li>Solution : Use collision avoidance protocol </li></ul><ul><ul><li>Two method like operations </li></ul></ul><ul><ul><ul><li>Ethernet-like </li></ul></ul></ul><ul><ul><ul><li>MACAW </li></ul></ul></ul>
  27. 27. CSMA/CA Ethernet-like Comparison of MAC protocols in wired systems and wireless systems <ul><li>Similar to non-persistent ALOHA </li></ul><ul><ul><ul><li>there random time was used before re-sensing the channel when busy </li></ul></ul></ul><ul><ul><ul><li>exponential backoff is used </li></ul></ul></ul><ul><li>Nowadays most MAC are reprogrammable by users </li></ul><ul><ul><ul><li>vulnerable to cheating !!! </li></ul></ul></ul>
  28. 28. Comparison of MAC protocols in wired systems and wireless systems Fig. : IEEE 802.11 Medium Access Control Logic
  29. 29. CSMA/CA- MACAW Comparison of MAC protocols in wired systems and wireless systems <ul><li>MACAW : send RTS/CTS packets to see whether you should defer </li></ul><ul><li>transmission to avoid interference with another transmission </li></ul><ul><li>Evolution of MACA </li></ul><ul><li>Network Allocation Vector (NAV): it’s a virtual channel that a station </li></ul><ul><li>assigns to itself telling it to shut up!!! </li></ul><ul><li>Note: hidden station problem is solved but NOT exposed station problem! </li></ul>
  30. 30. CSMA/CA- point Coordination Comparison of MAC protocols in wired systems and wireless systems <ul><li>In PCF the base-station polls the other stations, asking them if they have anything to send </li></ul><ul><li>It sends a beacon frame once every 10 or 100 ms </li></ul><ul><li>Sleep state to save battery </li></ul><ul><li>When base station transmits, there can be no hidden terminals </li></ul><ul><li>PCF and DCF can co-exist together </li></ul><ul><ul><li>By carefully defining the inter-frame time interval </li></ul></ul><ul><ul><li>First the base station can poll the other stations </li></ul></ul><ul><ul><li>I f nobody replies, any station can acquire the channel </li></ul></ul>
  31. 31. Comparison of MAC protocols in wired systems and wireless systems Fig. : IEEE 802.11 Protocol Architecture
  32. 32. 802.11 MAC Frame Layout Comparison of MAC protocols in wired systems and wireless systems <ul><li>Type : Data, control, or management frame </li></ul><ul><li>Subtype : Are we dealing with RTS, CTS, an ACK, etc </li></ul><ul><li>DS : Is the frame entering/leaving the current cell? </li></ul><ul><li>MF : Frames are allowed to be fragmented to increase reliability. This bit tells whether more fragments are on their way </li></ul><ul><li>Retry : Is this a retransmission? </li></ul><ul><li>O : Stick to ordered delivery of frames </li></ul>
  33. 33. 802.11 MAC Frame Layout Comparison of MAC protocols in wired systems and wireless systems <ul><li>Duration : Tells how long the transmission of this frame will take, allowing other stations to set their NAV accordingly </li></ul><ul><li>Addresses : Source/destination in a cell; and those of base stations outside the cell when dealing with inter-cell traffic </li></ul><ul><li>Sequence : Sequence number of this frame. 4 bits are used to </li></ul><ul><li>identify a fragment of a frame </li></ul>Note : 802.11 supports encryption at MAC level, Ethernet does not !!!
  34. 34. Broadband Wireless Comparison of MAC protocols in wired systems and wireless systems Goal : Use wireless connection between buildings i.e. avoiding the use of the local loop <ul><li>More bandwidth is needed: 10-to-66 GHz frequency range </li></ul><ul><li>Full-duplex communication possible </li></ul><ul><li>M uch of the mobility stuff from 802.11 is not needed </li></ul><ul><li>Can be focused in directional beams ( IEEE 802.11 is omnidirectional ) </li></ul>WiMAX (World Interoperability for Microwave Access) is a family of IEEE 802.16 aiming at replacing ADSL and cable modems
  35. 35. Broadband Wireless Comparison of MAC protocols in wired systems and wireless systems <ul><li>MAC : the base station controls the systems by scheduling downstream and upstream channels </li></ul><ul><ul><ul><li>it is connection-oriented ( QoS needed by phone-companies) </li></ul></ul></ul><ul><li>Time Division Duplexing (TDD) </li></ul><ul><ul><ul><li>Downstream traffic is mapped onto time slots by the base station </li></ul></ul></ul><ul><ul><ul><li>Upstream traffic is more complex </li></ul></ul></ul><ul><ul><ul><ul><li>The base-station pre-allocates it ( constant bit rate service ) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>The base-station periodically polls stations ( variable bit rate service ) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>No polling but subscribers have to contend ( best-effort service ) </li></ul></ul></ul></ul>Fig. : TDD frame contatinig time slots
  36. 36. Broadband Wireless- 802.16 Frame Structure Comparison of MAC protocols in wired systems and wireless systems Fig. : (a) a generic frame (b) a bandwidth request frame <ul><li>Connection-oriented Service </li></ul><ul><li>Checksumming optional </li></ul><ul><li>Encryption is critical for the system </li></ul><ul><ul><li>Managed at MAC level </li></ul></ul>
  37. 37. Wireless Sensor Networks Zigbee Comparison of MAC protocols in wired systems and wireless systems <ul><li>IEEE standard 802.15.4 </li></ul><ul><li>N ame of a specification for a suite of high level communication protocols using small, low-power digital radios </li></ul><ul><li>MAC layer: CSMA/CA </li></ul>
  38. 38. Conclusion Comparison of MAC protocols in wired systems and wireless systems
  39. 39. Comparison of MAC protocols in wired systems and wireless systems Fig: Channel allocation methods and systems for a common channel
  40. 40. Conclusion Comparison of MAC protocols in wired systems and wireless systems <ul><li>Simplest allocation schemes: FDM and TDM </li></ul><ul><ul><li>Poor choices for large, variable or bursty traffic </li></ul></ul><ul><ul><li>Alternative: ALOHA (Dynamic Allocation) </li></ul></ul><ul><li>Carrier Sensing in LANs and MANs led to variety of protocols </li></ul><ul><ul><li>Binaray Countdown </li></ul></ul><ul><ul><ul><li>Eliminates contention completely </li></ul></ul></ul><ul><ul><li>Tree Walk </li></ul></ul><ul><ul><ul><li>Reduces contention by dividing stations dynamically </li></ul></ul></ul><ul><li>Ethernet </li></ul><ul><ul><li>Dominant in Wired LANs </li></ul></ul><ul><ul><li>Uses CSMA/CD </li></ul></ul>
  41. 41. Conclusion Comparison of MAC protocols in wired systems and wireless systems <ul><li>Wireless LANs </li></ul><ul><ul><li>CSMA does not work here </li></ul></ul><ul><ul><li>MACA and MACAW </li></ul></ul><ul><ul><li>FHSS and DSSS </li></ul></ul><ul><ul><li>IEEE 802.11 suggests CSMA/CA </li></ul></ul><ul><li>Broadband Wireless </li></ul><ul><ul><li>802.16, TDD </li></ul></ul><ul><li>Wireless Sensor Networks </li></ul><ul><ul><li>Zigbee </li></ul></ul><ul><ul><ul><li>CSMA/CA </li></ul></ul></ul>
  42. 42. References <ul><li>Andrew S. Tanenbaum, Computer Networks, Fourth Edition </li></ul><ul><li>William Stallings, Data and Computer Communications, Eighth Edition </li></ul><ul><li>Holger Karl, Andreas Willig, Protocols and Architectures for Wireless Sensor Networks </li></ul><ul><li>Paolo Costa, Compter Networks- Medium Access Control Sublayer </li></ul><ul><li>Alaa Muqattash, Marwan Krunz, CDMA-Based MAC Protocol for Wireless Ad Hoc Networks </li></ul><ul><li>Sushant Jain, Ratul Mahajan, Wireless LAN MAC protocols </li></ul>Comparison of MAC protocols in wired systems and wireless systems
  43. 43. Thank You!!! Any Questions??? Comparison of MAC protocols in wired systems and wireless systems

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